This invention relates generally to high energy confined plasmas and more particularly is directed to measuring the velocity distribution of confined energetic alpha particles resulting from deuterium-tritium fushion reactions in a confined energetic plasma.
Significant quantities of energetic alpha particles produced by deuterium-tritium fusion reactions are expected to be produced in the next generation of magnetic confinement fusion devices such as the Tokamak Fusion Test Reactor (TFTR) and the Joint Experimental Tokamak (JET). It is also expected that substantial heating of the plasma by the deuterium-tritium fusion-product alpha particles will occur as they slow down classically. How well the energetic alpha particles are confined within the plasma will be one of the most significant questions to be answered. Currently under study are various anomalous processes which could alter the slowing down of the alpha particles and their energy transfer characteristics to the plasma. These anomalous processes could lead to loss of the fast alphas from the central portion of the plasma or these processes could perhaps change the rate at which the fast alpha particles slow down and heat the ions and electrons in the plasma so as to sustain the fusion reaction. The anomalous loss of alpha particles from the central portion of the plasma being heated to ignition could increase the n.tau..sub.E, the auxiliary heating power, and the .beta. required for ignition. Anomalous ion heating by alpha particles could lead to a reduction in ignition requirements. In addition, the loss of alpha particles from the plasma could result in accelerated erosion of the reactor first wall from blistering due to bombardment by the escaping energetic alpha particles.
Given the large costs and lead times associated with reactor-sized expriments, it is desirable to determine as early as possible whether the fusion-product alpha particles slow down in a classical manner through binary coulomb collisions or whether they instead are subject to anomalous processes prior to thermalization.
The first generation of deuterium-tritium burning tokamak devices is unlikely to provide answers regarding alpha particle confinement through the power balance characteristics alone of these devices. At Q=1, alpha particle heating will account for only about 1/5 of the input power to the plasma, even if the alpha particles thermalize completely before they are lost. Because of the importance of understanding alpha particle behavior in a fusing plasma, a diagnostic is needed to measure the slowing down spectrum of the confined alpha particles.
Therefore, the present invention provides a means and method for providing an accurate analysis of the behavior in a magnetically confined plasma of the deuterium-tritium fusion-product energetic alpha particles. The present invention involves measuring the velocity distribution of the confined energetic alpha particles by seeding the fusion plasma with a stable element which could undergo nuclear reactions with the alpha particles to produce radioactive product nuclei. A fraction of these product nuclei are then captured by a probe at the edge of the plasma, which is subsequently withdrawn to a low-background area to count the decays. This radiochemical technique makes use of generally developed techniques and does not require a great deal of development and expense.